1. Field of the Invention
The present invention relates to an electronic device having an element that is comprised of a light emitting material sandwiched between electrodes, and to an electric apparatus using the electronic device for its display unit (display or display monitor). Specifically, the present invention relates to an electronic device using a light emitting material that provides EL (Electro Luminescence) (Note that the material will hereinafter be called EL material).
2. Description of the Related Art
In recent years, development is proceeding in an electronic device using a self light emitting element that utilizes the EL phenomenon of a light emitting material (hereinafter referred to as EL element) (the device will hereafter be referred to as EL display device). The EL display device is a display device that uses a self light emitting element and, hence, unlike a liquid crystal display device, does not need a backlight. In addition, the EL display device has a wide angle of view, which makes the device a promising candidate for a display unit of a portable apparatus for outdoor use.
There are two kinds of EL display device: a passive type (passive matrix type) and an active type (active matrix type), and both types are being developed actively. However, what draws attention most is, at present, an active matrix type EL display device. The EL material emitting EL and forming a light-emitting layer also is divided into two types, one being an organic EL material and the other being an inorganic EL material. The organic material is further divided into a low molecular weight type (monomer type) organic EL material and a high molecular weight type (polymer type) organic EL material. The polymer type organic EL material particularly highly regarded, or it is easier to handle and has higher heat resistance in comparison with the low molecular weight type organic EL material. Incidentally, a light-emitting device using an organic EL material is called OLED (organic light emitting diode) in Europe.
The active matrix type EL display device is characterized in that each of pixels that constitute a pixel portion is provided with an electric field transistor, recently, a thin film transistor (hereinafter referred to as TFT), to control the amount of current flowing through an EL element by the TFT. As a typical pixel structure for such an active matrix type EL display device, there is known a structure illustrated in FIG. 1 attached to Japanese Patent Application Laid-open No. Hei 8-241048.
The pixel structure disclosed in the publication sets two transistors (T1, T2) in one pixel, and a capacitor (condenser: Cs) is provided in a drain of the transistor (T1) parallel to the transistor (T2). This capacitor (condenser) is necessary for holding a voltage applied to a gate of the transistor (T2) for one field period or one frame period.
When two transistors and a capacitor (condenser) are formed in one pixel however, these elements occupy almost all the pixel area, causing a reduction of the effective light emission area (the area in which light emitted from a light-emitting layer is allowed to transmit to be used).
The present invention has been made in view of the above problem, and an object of the present invention is therefore to provide an electronic device capable of bright image display by using a pixel structure with a large effective light emission area. Another object of the present invention is to provide an electronic device of high reliability. Still another object of the present invention is to provide an electric appliance using the electronic device as its display unit.
Yet still another object of the present invention is to provide a process for reducing the cost of manufacturing the electronic device capable of displaying an image with high brightness.
The present invention is characterized in that a voltage applied to a gate of a TFT for supplying current to an EL element (hereinafter referred to as current controlling TFT) is held by a gate capacitor (parasitic capacitor formed between the gate and an active layer) of the current controlling TFT. In other words, the present invention actively utilizes the gate capacitor of the current controlling TFT (corresponding to the transistor (T2) in FIG. 1 of Japanese Patent Application Laid-open No. Hei 8-241048) instead of the capacitor (condenser: Cs) shown in FIG. 1 of Japanese Patent Application Laid-open No. Hei 8-241048.
The present invention is characterized in that an LDD region is formed on a drain region side of the current controlling TFT that is comprised of a P-channel TFT so that the LDD region overlaps with a gate electrode through a gate insulating film sandwiched therebetween. Usually, a P-channel TFT is used without forming therein any LDD region, and thus the invention is characterized by forming the LDD region in order to form the gate capacitor.
The structure as such practically dispenses with the area the capacitor (condenser) occupies, thereby greatly increasing the effective light emission area.
The EL display devices referred to in this specification include triplet-based light emission devices and/or singlet-based light emission devices.
The present invention employs a process of manufacturing a plurality of electronic devices from one large-sized substrate in order to reduce manufacturing cost of the electronic device, namely, to produce a low cost electronic device. The characteristic of the present invention resides in considerably reducing the manufacturing cost by limiting the investment in plant and equipment to a minimum with employment of a process to which an existing production line for liquid crystal can be applied.